FIG. 1a shows a typical high voltage NPN transistor which has a heavily N+ doped emitter, a moderately P doped base region and a lightly N doped collector, with a Heavily N+ doped collector ohmic contact.
With a high voltage on the collector, the collector-base junction is reverse-biased, and a charge depletion region extends largely into the collector due to its lower doping level. If a forward current is made to flow in the base-emitter junction, most of the current consists of electrons flowing from emitter to base, with a small current component of holes flowing from base to emitter, due to the higher N+ doping of the emitter relative to the P doping of the base.
With a thin base, most of the electrons from the emitter flow through the base region and into the collector, causing a collector current to flow which is typically much larger than the base current. As long as the collector voltage is sufficiently high the transistor operates in the “unsaturated” region of the characteristic curves of FIG. 1b, where the collector/base current ratio is approximately constant. Only electrons are injected into the collector from the emitter. As the collector is N doped these electrons are “majority” carriers, and collector current can change very rapidly in response to base current.
As the collector voltage reduces with current still flowing, the depletion region disappears, and a point is reached where the collector-base junction becomes forward biased (quasi-saturation begins). At high currents there will still be significant voltage (perhaps tens of volts) on the external collector contact due to the resistive I×R drop in the lightly doped region. The forward biased collector-base junction now begins to inject holes (minority carriers) into the lightly doped collector.
The techniques we describe later are particularly relevant to power semiconductor devices; when we refer to power devices we are generally referring to high voltage devices which typically operate with a voltage of greater than 50 volts, more usually greater than 100 volts or 200 volts. Generally (but not always) the devices operate at powers of greater than 1 watt. As described above, broadly speaking in a high voltage device (and also, less noticeably, in a low voltage device) the extended collector connection has a resistance and the bipolar transistor shown in the symbol of FIG. 1a can be considered to have a (non-linear) resistor in series between the device itself and the collector terminal. Effectively the internal device can be in saturation (for example, the base-collector junction being forward biased) whilst externally the collector-emitter voltage drop is governed by the voltage across the internal non-linear resistor (the resistance of which falls as the current rises due to the injection of carriers).
The presence of holes in the collector increases the electron density to maintain approximate charge neutrality, and the resulting electron-hole plasma lowers the collector resistance, and hence the voltage drop. If the base current is increased further the collector resistance drops roughly as the inverse of the base current, until eventually a point is reached where fixed resistances become dominant. Still higher base currents increase the stored charge in the collector region, but the collector-emitter voltage falls no further and the transistor is in full or deep saturation.
Turning off the bipolar transistor generally entails removal of all the charge stored in the collector. Two methods can remove charge from the bipolar's collector region:    1. Internal base-emitter current (charge supplied by the collector to maintain conduction);
Removal of forward base current from a saturated BJT stops the injection of minority charge into the collector, and the stored collector charge now flows into the emitter through the base, sustaining conduction. This internal base current is amplified by the current gain of the transistor, and is also a slow mechanism for turn-off.    2. External reverse base current;
Large reverse base currents are the most effective way to turn off BJTs quickly, particularly in conjunction with stored charge minimization schemes (such as avoiding deep saturation). There are various limitations on the amount of reverse base current that can be drawn, such as lateral base resistance, base-emitter junction breakdown voltage, and charge drift velocity from the collector into the base.
One technique for regulating the base current for switching a bipolar transistor is described in U.S. Pat. No. 5,017,802 but this circuit has the drawback that it is affected by the aforementioned (variable) collector resistance. It is also known to use a Baker clamp (a diode between the base and collector of a transistor) to prevent deep saturation when the transistor is driven hard, providing a path for excess base current, thus speeding up the response time of the transistor. However when a transistor is in quasi-saturation as described above the collector voltage can be greater than the base voltage and this type of clamp can fail.
Broadly speaking we will address the problem of controlling the base drive current in order to avoid excess stored charge at the desired instant of turn-off, with the aim of minimising turn-off time. The techniques we describe are particularly useful in power converters but are not restricted in their application to such circuits. This is because a power converter presents a combination of potentially conflicting requirements—use of a high voltage device with a low on resistance (for low loss and hence increased efficiency) and a fast switch-off—for example the circuit may be operating at, say, above 100 KHz.